Ventricular arrhythmias are a major source of morbidity and mortality in this country. Pharmacologic treatment of these arrhythmias is often unsuccessful, and there is growing interest in surgical and catheter techniques at eliminating them. Recently, signal-averaged electrocardiography has been used to demonstrate abnormal deflections occurring at the terminus of the QRS complex in many patients with ventricular arrhythmias; these are known as "late potentials." Late potentials are thought to arise from localized areas of slowly-conducting myocardium, which is thought also to generate localized fractionated electrograms recordable directly on the cardiac surface. The extent of fractionated electrograms has been shown to correlate with the risk of arrhythmias, and the location of fractionated electrograms has been shown to correlate with areas of arrhythmogenic myocardium. Previous methods of detecting late potentials have not attempted to define their location on the body surface. Preliminary studies in a canine model of myocardial infarction from this laboratory have suggested that late potentials are localized to a contiguous area on the thorax. The primary hypothesis that this application proposes to study is that the location of late potentials indicates the epicardial location of fractionated electrograms and slow conduction, which in turn can guide ablative therapy of arrhythmias. This hypothesis will be examined both in a canine model of healed myocardial infarction and in patients with healed myocardial infarction. Substantiation of this hypothesis would allow for noninvasive determination of the extent and location of arrhythmogenic myocardium in patients with ventricular arrhythmias, which may help direct surgical and catheter ablation.